Global Ecology 3 2 – 1
Chapter
32
Global Ecology
Drunken forest

As permafrost melts, a process now
occurring at an increased rate because of climate change, trees
formerly anchored in the icy soil shift and tilt, as seen in this
“drunken forest” of black spruce near Fairbanks, Alaska. Organic
material trapped in the ice for thousands of years decays,
releasing enormous amounts of carbon dioxide and methane,
gases that trap heat and warm the planet.
chapter outline
Life on the Land
Rainforests
Deciduous Tropical Forests
Savannas
Deserts
Grasslands
Temperate Deciduous Forests
Temperate Mixed and Coniferous Forests
Mediterranean Scrub
The Northernmost Forests—Taiga and Boreal Forest
Arctic Tundra
A Final Word
T
o give a proper context for a broad view of world vegetation, we first zoom in to look at two small patches: one in
the wet tropics and one in the north temperate zone. Consider
first an area measuring 10 meters by 10 meters in the tropics.
As we know, ecosystem function depends on the trapping of
solar energy and its conversion into plant biomass, which then
feeds not only the higher trophic levels but also the decomposers that digest dead biomass. The solar energy that drives
photosynthesis is, however, only a tiny fraction of the energy
picture. Most of the solar energy that reaches this tropical patch
provides the heat that keeps it in the range of 20° to 35°C, driving the evaporation and transpiration (the “evapotranspiration”)
that release water vapor into the air. Since we are in the wet
tropics, the precipitation input exceeds the evapotranspiration
32–1
output of this small area, and the excess water runs off into rivers and streams. We also know that this patch will experience
only moderate climate shifts from day to day, week to week,
and month to month. Much of the climate variability in this
ecosystem relates to the cycle of day and night, and much less
to the seasons, which vary little in the wet tropics.
Now let’s look at a patch of the same size in the moist
temperate zone of North America. We find, as in the wet tropics, that precipitation exceeds evapotranspiration and that infiltration and runoff feed streams and rivers. But in contrast, there
CHEC K POINT S
After reading this chapter, you should be able to answer the following:
1. What is a biome, and what factors affect the distribution of
biomes on Earth?
2. Given that tropical rainforests contain such a diversity of species,
why are many kinds of tropical soils unsuitable for agriculture?
3. Which factors favor grasslands over wooded ecosystems? What
role does fire play in grasslands?
4. Which best characterizes a desert: high temperature or low
precipitation? How have plants adapted to desert living?
5. How does the appearance of the temperate deciduous forest
biome change from one season to the next? What accounts for
these changes?
6. What is the general global pattern for biodiversity, and what
factors might explain it?
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C ha p t e r 3 2 Global Ecology
are large differences between seasons, a variation that is crucial
to the functioning of this ecosystem. The seasons range from
long, hot days in midsummer to short, often bitterly cold days
in winter. During the winter, photosynthesis is either low (as for
an evergreen spruce tree) or completely lacking (as for a deciduous maple tree). The growth that occurs between the last killing
frost of the spring and the first killing frost of the autumn is substantial. It even equals, in some cases, that of the continuously
productive tropics.
Clearly, then, what happens locally depends on global patterns. Starting with energy, on any single day when plants are
growing, the amount of solar radiation received that is in the
restricted wavelengths active in photosynthesis (Chapter 7 of
the textbook) sets an upper limit to productivity. The temperature of the air and of the soil must remain in a range that allows
metabolic functioning. The soil moisture also sets a maximum
for productivity. Without adequate water supplies, the photosynthetic machinery will slow or even shut down entirely. For
rainfall to exceed evapotranspiration, the extra water must come
from somewhere outside a local patch. And so we need to consider how moisture moves from places with more moisture—the
oceans—to places with less. To understand any locality, we must
understand the functioning of the global climate machinery by
which heat and moisture are redistributed around the globe.
But many other factors besides regional climate come
into play in shaping a local ecosystem. If we were to shift our
10-square-meter patch by 100 meters in any direction, we would
not find exactly the same collection of organisms. Usually, we
would have some, perhaps most, of the species we had at the
first location. But they might also be quite different. The changes
we would see in this 100-meter shift would not be related to the
regional solar radiation or rainfall. Rather, they would depend on
many local factors such as the nature of the substrate, associated
species, the slope of the land (which affects the solar radiation
received), and the presence or absence of standing water. It would
also be affected by the history of extreme events, such as fire,
windthrow, or clearing of the land for agriculture. Suppose, for
example, that our 100-meter shift in the temperate forest crosses
a geologic boundary—from soil formed in glacial till, nutrientrich and high in the finer soil particles (silts and clays) that retain
moisture, to a coarse, deep outwash sand, nutrient-poor and subject to drought. We would expect the plant communities to be different in these two places, even though the climate would be the
same. Scientists who try to explain why vegetation in one place
differs from that in another are very aware that some differences
require attention to local factors, others to global factors.
Despite the local variation, there are repeating patterns on
the global scale. The similarity between two patches of vegetation in different parts of the world can arise in two ways. Species
that are related by descent (they share a near ancestor) will, in
general, look similar and function in a similar way. Species that
are distantly related (they diverged from their common ancestor
a long time ago) will be distinctly different genetically, but nevertheless may be similar in appearance and function. Consider
deciduous forests in Europe and in the eastern United States.
Many of the genera are the same—for example, oaks (Quercus),
beeches (Fagus), and ashes (Fraxinus). Most of us, parachuted
into the middle of such a forest without knowing which continent we were on, would not be able to deduce where we were
from the general look of the vegetation. In this example, the
abundant species are related by descent and are also morphologically and physiologically similar, because the climate and soils
are very similar. We can repeat the experiment by dropping into a
high-rainfall tropical area in South America or into an area with
a similar climate in the Indonesian archipelago. The species and
genera would mostly be different in these two areas, but unless
you knew tropical rainforest taxonomy, the general appearance of
the vegetation would give few clues to your geographic whereabouts. Similarities that arise despite genetic differences evolve
because similar habitats favor similar morphologies and physiological adaptations (Figure 32–1). The process by which natural
selection produces similarity in unrelated species is called convergent evolution (page 239).
But there are changes over time as well as across space.
If we revisit a 10-square-meter plot after leaving it for a day,
we expect it to be much the same but not absolutely identical
in every detail. If we revisited the site a year, 10 years, or 100
years later, we would not be surprised to find major changes. In
a region subject to fire, we might find a forest recovering from a
wildfire, seeing only young trees and shrubs. If we came back a
million years later, a formerly dry site might now be under water,
and even if the substrate were similar, new species would probably have evolved. These kinds of changes over time are another
source of variation. In our modern world, changes caused by
humans have become all-pervasive and often drastic. A forest can
be replaced by a shopping center.
Armed with these concepts, we begin our survey of world
vegetation from what we might call the 3000-meter level—as
viewed from an imaginary blimp flying over the globe and traversing the major gradients: tropics to temperate to Arctic, wet
to dry, and low elevation to high elevation. Thanks to the Google
Earth website, we can take such a virtual journey. It allows us to
locate nearly any place on Earth and zoom in and out to view it
on different scales (see the essay “Google Earth: A Tool for Discovering and Protecting Biodiversity” on page 241). If you do
this, you will get a sense of the pervasiveness of human activity.
There are places (such as Antarctica and the driest deserts) where
human influence is not apparent, but overall, your virtual world
tour will clearly show that humans have substantially modified
the Earth. This means that a study of the Earth’s present ecosystems must include an understanding of the ecology of human
land use and of the interactions between humans and nature.
For more than 99.9 percent of the Earth’s history, though, there
were no humans. And although other organisms have affected
their environments—consider how photosynthesizers oxygenated
the atmosphere—none have caused such drastic change so rapidly. It is now estimated that 40 percent of the Earth’s surface is
devoted to agriculture, and much of the remaining 60 percent has
been significantly altered by humans—just one example being
the cutover forests of the United States. Understanding what
the world’s landscapes looked like before their modification by
humans is important if we are to understand our contemporary
biosphere and, most importantly, if we are to grasp what we may
lose if we fail to take steps to save this rich biological heritage.
Life on the Land 3 2 – 3
(a)
(b)
32–1 Convergent evolution at high elevations (a) Espeletia
pycnophylla growing in the high-altitude ecosystem known as the
páramo in the Andes, as seen here in Ecuador, South America.
(b) Dendrosenecio adnivalis growing at a high elevation on Mount
Gessi in Tanzania, Africa. The unique challenges of a habitat that
freezes almost every night and warms in the day favor a compact
tree form with robust leaves and well-protected growing points.
These similar forms evolved independently on two distant
continents, a striking example of convergent evolution.
How can we determine what the Earth was like before the
wholesale modifications brought about by our species? It is often
possible to see the signature of the original vegetation. Imagine flying our blimp from coast to coast over the United States,
starting in New York and heading west. You notice that until you
cross the Mississippi River, uncultivated and minimally managed lands are usually forested with broad-leaved deciduous
trees. If very wet, these areas are covered with marsh. Crossing
central Illinois, you may not see clear evidence that in 1800, this
area was mostly grassland, because most of the land is cultivated. As you head farther west, however, you see that uncultivated lands support fewer trees, except along rivers and streams,
and it becomes obvious that grassland is the dominant natural
vegetation.
Over the Rocky Mountains, you cross from grasslands back
into forests as the elevation increases, but now the dominant
trees are evergreen conifers, though there will be grassy valleys and areas of treeless alpine tundra. On the western side of
the Rockies in Colorado, with the land dropping in elevation and
with dryness increasing, you see vast expanses of shrubby grasslands punctuated with forested hills. Crossing the Sierra Nevada
in California, you encounter a complex landscape of coniferdominated mountain slopes, open oak woodlands, grasslands,
and shrub communities of chaparral and coastal sage. Over the
Central Valley you see one of the world’s most productive agricultural areas and little trace of the formerly extensive marsh
areas drained to create it. Completing your trip on the beach in
Santa Monica, California, you may wonder how to describe and
explain the variation you have seen.
To answer this question, we employ the concept of the
biome, which we define as a type of vegetation that is reasonably homogeneous over large terrestrial areas with respect to the
growth forms, sizes, and arrangement of the dominant plant species. Each biome also has an associated suite of animals. Like
humans, they affect vegetation, but in general they are not defining characteristics of the biome.
Life on the Land
The scheme we present for classifying the world’s natural vegetation into a set of biomes is based on the work of the late A.
W. Küchler, of the University of Kansas (Figure 32-2). This
scheme is, of course, a broad-brush generalization. Making a
map of this kind requires delineating areas. But a line on a map
does not usually correspond to an easily discernible “line” on the
ground. Similarly, the area inside the lines includes a lot of variation, and the larger the area of the unit, the greater the variation
within. For example, biome 5 on the map—alpine tundra and
mountain forests—encompasses continuous forest and treeless
alpine tundra. Not mentioned, but also included in this biome,
are mountain meadows covered with grasses, sedges, and other
herbaceous plants, with no trees, or only widely scattered ones.
Mapped biomes generally contain very different communities
within their boundaries.
What are the characteristics of the biomes, passing from the
poles to the equator? By ignoring the often strong local effects
of elevation (mountains) and moving only through regions with
plenty of moisture during the growing season, we can attribute
the dramatic changes to differences in temperature that, in turn,
depend primarily on the amount of solar radiation received,
which is largely explained by latitude. It is the geometry of the
Earth and its path through space with respect to the sun that creates the relationship between latitude and solar radiation. First
consider the effects of the Earth’s nearly spherical shape. Imagine
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C ha p t e r 3 2 Global Ecology
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Taiga
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Monsoon forests
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that the Earth has an axis of rotation perpendicular to the plane
of its orbit. It does not, of course, because the Earth is tilted to
approximately 23.45°, but if it were perpendicular, the solar
radiation per unit surface area of the Earth would decrease with
increasing north and south latitude, because the same amount
of sun energy would be spread over an increasingly larger area.
Moreover, since the Earth has an atmosphere that can reflect and
absorb incoming energy, and the incoming solar radiation traverses a greater depth of atmosphere at higher latitudes before
reaching the surface, the amount of solar energy hitting the
ground is further diminished away from the equator.
Tilting the Earth’s axis to its correct angle of 23.45° and
setting the planet in motion around the sun, we can see that not
only do the same equator-to-pole patterns still apply, but as the
Earth travels around the sun, first the Northern Hemisphere and
then the Southern Hemisphere is “tilted toward the sun.” At
23.45° north latitude, for example, at the June summer solstice,
the sun at noon is directly overhead, and solar radiation per unit
17
32–2 Biomes of the world The
information in these maps and the
accompanying key was originally
supplied by A. W. Küchler of the
University of Kansas, one of the leading
authorities on the distribution of
biomes. Because of the global coverage
of the maps, the scale is relatively small
and the content is generalized. Any
given biome is not always uniform, and
all the biomes include considerable
variations in vegetation. The boundaries
between the biomes may be sharp, but
more often they are blurred, consisting
of broad zones of transition from one
type of vegetation to another.
area at the Earth’s surface is at its maximum—discounting the
possible effect of clouds and dust. On this same day, poleward
from 23.45° north, daylength increases, reaching 24 hours above
the Arctic Circle. In temperate zones, this seasonal cycle reveals
itself by the rising and falling of the path of the sun. Solar input
is at a maximum at the summer solstice and a minimum at the
winter solstice—a pattern well understood and carefully followed
even in preliterate antiquity.
Although temperature at any point is dependent on the solar
radiation received, this is significantly modified by the air and
water currents that redistribute heat. For the Earth as a whole,
energy absorbed in the warmer tropical regions is transferred to
higher latitudes, so that many parts of the globe are warmer than
the solar radiation received at their latitude would indicate. Study
of air and water movements has revealed large-scale global patterns (Figure 32–3) consisting of latitudinal circulation bands.
Within the equatorial zone (23.45° south to 23.45° north), the
heating promotes evaporation and transpiration, and warm, moist
Life on the Land 3 2 – 5
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air ascends. The rising air drops much of its moisture in the tropical zone, but it sets up a cyclical pattern, and the air, with much
of its moisture removed, tends to descend at about 30° north and
south. The descending air warms as pressure increases, causing
the relative humidity to decline even further and to produce a
stable atmosphere. The cyclical flow initiates winds that, because
of the rotation of the Earth, trend away from the equator-to-pole
flow to create the westerlies and trade winds (Figure 32–3).These
global patterns explain why the most extreme deserts in the
world are found around 30° north and south, and why the regions
of highest rainfall in North and South America are found at the
higher latitudes of their west coasts. Another factor that determines the aridity of a particular spot on land is how far it lies
from the moisture-rich air of the oceans.
The elevation of land—whether mountains or sea-level
plains—has a significant influence on local climate, and the
change in vegetation with increasing elevation can be dramatic
over very short distances. To explain this, we need to remember
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that it is primarily the absorption of short-wave energy at the
Earth’s surface that captures the heat energy of the sun. The sunwarmed surface radiates long-wave radiation back into the atmosphere. Some of this is dissipated into space, but much is returned
to Earth by the dust, water vapor, and carbon dioxide, as well as
other greenhouse gases, that absorb the re-radiated heat, warming
the air and the surface.
In general, air temperature decreases with increasing elevation above the ground. There are reversals of this usual trend,
called inversions, in which for some distance the air temperature
actually increases with altitude, but these are local and generally temporary exceptions. On average, air temperature drops by
about 6.4°C for each 1000 meters of elevation (3.5°F for each
1000 feet), with the observed rate of change depending on time
of day, moisture content of the air, wind patterns, and other topographic and weather variables. Because of the decrease in temperature with altitude, locations on the Earth that are topographically
higher have significantly cooler temperatures, on average, than
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C ha p t e r 3 2 Global Ecology
32–3 Global air currents The Earth’s surface is
covered by belts of air currents, which determine the
major patterns of wind and rainfall. In this diagram,
the blue arrows indicate the direction of movement
of the air within the belts. The green arrows indicate
regions of rising air, which are characterized by high
precipitation, and the brown arrows indicate regions
of descending air, characterized by low precipitation.
The dry air descending at latitudes of 30° north and
south is responsible for the great deserts of the
world. The prevailing winds on the Earth’s surface,
indicated by the black arrows, are produced by
the twisting effect of the Earth’s rotation on the air
currents within the individual belts.
Green arrows:
Moist air rises
and cools—
high precipitation
60° N
Westerlies
30° N
Northeast trade winds
0°
Direction of
Earth’s rotation
Equatorial doldrums
Southeast trade winds
30° S
Brown arrows:
Dry air descends
and warms—
low precipitation
Westerlies
60° S
locations at the same latitude at lower elevations. This effect
is slightly counterbalanced by the fact that the air is thinner
(because air pressure, and therefore density, also decreases with
elevation) and the path of sunlight through the absorbing atmosphere is somewhat shorter.
The cooling with increasing altitude has profound climatic
consequences when air masses move across mountains. Because
cool air cannot contain as much moisture as warm air, moisturerich air masses produce precipitation as they are carried upward.
The highest rainfall on Earth occurs in places where this effect
is particularly strong. For example, Mount Waialeale on Kauai,
Hawaii, receives moisture-laden air driven by the northeast trade
winds. The annual precipitation at the highest elevations (1554
meters) on the windward (north and northeast) slopes averages
about 9.5 meters (31 feet). This is extreme, but we typically
find wetter climates on mountains’ windward slopes than on the
adjacent lowlands or on the leeward slopes. As the air descends,
it warms, and its capacity to contain moisture correspondingly
increases, making rainfall less likely. For example, the leeward
side of Mount Waialeale receives only about 50 centimeters of
annual precipitation. This downwind dryness is the “rain-shadow”
effect (Figure 32–4). It is particularly strong along the Pacific
coast of North America, with zones of aridity or semiaridity on
the east side of mountain chains, from the northern United States
south into Mexico. In South America the pattern is also strong,
except that in northern South America the rain shadow is on the
western side of the Andes, and in southern South America (Patagonia) the rain shadow is on the east—a pattern explained by the
shift in prevailing wind direction (Figure 32–3).
The climatic changes that you observe when climbing a
mountain mimic the changes that occur as you travel toward the
poles from the equator—mainly with respect to temperature and
the occurrence of freezing conditions (Figure 32–5). An increase
in elevation of 100 meters (328 feet) produces a change in mean
temperature corresponding to an increase in latitude of approximately 1 degree. Climbing a tall mountain can take you through
a sequence of ecosystem types that resembles the sequence you
might encounter if you went north toward the Arctic. For example, in the southwestern United States, the highest mountains are
topped by conifer-dominated forests, then, at higher elevations,
by tundra-like meadows and scree fields, and eventually by a
zone of perpetual snow cover. But this “replicate the latitudinal
Descending,
warming air
Rising,
cooling air
Rain shadow
Prevailing wind
32–4 Rain shadow The effect of coastal mountains on patterns of
precipitation in the Northern Hemisphere. As winds come off the
water, the air is forced upward by the contour of the land, cools, and
releases its moisture in the form of rain or snow. As the air descends
on the far side of the mountains and becomes warmer, its capacity to
hold water increases, so the amount and frequency of precipitation
decrease.
Rainforests 3 2 – 7
Snowcapped
peak
lin
Snow
e
Tundra
l in
Tree
Altitude →
e
Taiga
Temperate
deciduous forest
Polar
ice
Tropical
rainforest
Equator
Latitude →
Pole
32–5 Relationship between altitude and latitude In the Northern Hemisphere, the sequence of
vegetation changes we would see when ascending a tall mountain is similar to the sequence we
would see while traveling hundreds of kilometers to the north. Alexander von Humboldt was the
first to point out the relationship between altitude and latitude. To experience a similar sequence
of vegetation in the Southern Hemisphere, we could ascend a mountain. However, by simply
traveling south, we would never encounter vegetation corresponding to the taiga and the tundra
of the Northern Hemisphere. Can you explain why?
change” pattern becomes less true as one moves into the tropics.
On a tropical mountain, the climatic changes as you ascend are
dramatic, but the vegetation of the colder upper elevations does
not look much like what you would see in Alaska or Siberia.
A key difference is that tropical mountains have the same limited seasonality as the lowland rainforests. As a result, the daily
temperature variations are much greater than the seasonal average daily temperature extremes. As the ecologist O. Hedberg
put it, high-elevation tropical systems have “summer every day
and winter every night.” The demands of such extreme diurnal
temperature variation have favored the evolution of bizarre plant
types, and the high-altitude tropical mountain communities provide some of the most striking examples of convergent evolution
(Figure 32–1).
Soils and Fire Influence Regional Patterns of Distribution
Variation in terrestrial vegetation is often explained by variation
in the soil. Soil is best viewed as a sub-ecosystem, because it is
not just ground-up rock and dead organic matter but a complex
community of highly specialized organisms (see Figure 29–6).
It is the functioning of this system that largely determines the
abundance and chemical form of the elements made available to
plants. It also affects the amount and distribution of soil moisture, which for most soils is strongly dependent on the sizes of
the mineral particles, whether mostly microscopic clay particles or coarse sand and gravel. The kind of soil present within
a region is partly determined by climate but also by the “parent
material” in which the soil forms. Another factor is the length
of time a region has been stable and therefore has had surfaces
exposed to the processes of soil weathering. Like ecosystems
generally, the soil is not a static system. Australia, for example,
has been geologically stable over millions of years, and largely
because of this, the continuous leaching by precipitation has
produced nutrient-poor soils, with deficiencies of essential elements, especially phosphorus.
Fire also strongly affects vegetation. Before humans became technologically sophisticated in their destruction of the
natural world, fire was the major disturbance—where disturbance is defined as a factor that kills or significantly harms the
organisms in an ecosystem. Fire is only possible with fuel (combustible plant material) that is dry enough to burn, and the presence of dry fuel is strongly influenced by climate (for example,
whether or not there is a dry season) and by weather (such as
wind, the pre-fire pattern of precipitation, and lightning). Thus
fire, weather, and climate are intimately connected. Climate
determines the plant types present, the overall conditions that
control the death of plant parts, and weather conditions at the
time of the burn, such as the moisture in the fuel, the humidity
of the air, and, especially, the direction and strength of the wind.
Few places are absolutely free of the possibility of fire, but those
with the highest vulnerability combine a reliable moist growth
period to build up biomass with annual episodes of drought to
dry it out. These conditions exist over huge areas in the dry tropics and semiarid temperate zones. The boreal forest, though not
thought of as an arid region, dries sufficiently and has the right
fuel characteristics to permit huge wildfires. There is consensus
among ecologists that if, hypothetically, it were possible to prevent all fires, the vegetation of many parts of the world would
look very different. We discuss some of these situations in the
following sections.
Rainforests
We begin our exploration of biomes with the system where the
fewest factors are limiting: the rainforests—where temperatures
are relatively constant and never below freezing, where rainfall is
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A l e x and e r v o n H u mb o ldt
Alexander von Humboldt (1769–1859) was
perhaps the greatest scientific traveler who
ever lived and was certainly one of the greatest writers and scientists of his era. A native of
Germany, Humboldt traveled widely across
the interior of Latin America from 1799 to
1804 and climbed some of its highest mountains. Exploring the region between Ecuador and central Mexico, he was the first to
recognize the incredible diversity of tropical
life and, consequently, the first to realize just
how vast a number of species of plants and
animals there must be in the world.
In his travels, Humboldt was impressed
with the fact that plants tend to occur in repeatable groups, or communities, and that
wherever there are similar conditions—relating to climate, soil, or biological interactions—similar groupings of plants appear. He
also discovered a second major principle: the
relationship between altitude and latitude. He
found that climbing a mountain in the tropics is analogous to traveling farther north (or
south) from the equator. Humboldt illustrated
this point with his well-known diagram of the
zones of vegetation on Mount Chimborazo in
Ecuador, which he climbed. On this mountain,
he reached the highest elevation on record
for any human being up to that date.
On leaving Latin America in 1804, Humboldt visited the United States for eight
weeks. He spent three of these weeks as
Thomas Jefferson’s guest at Monticello, talking over many matters of mutual interest. It is
consistent and therefore soil moisture ample for growth throughout the year, where geology and topography allow deep soil formation and retention of at least an adequate supply of nutrients,
and where the site is well drained (that is, not under water at any
time). These conditions favor high and consistent photosynthesis
and maximum plant growth. Plants can expand their canopies
rapidly and, without the constraints of freezing and drought, can
have large and/or thin leaves for maximum absorption of light.
The result is strong competition for light. Plants that fall behind
in capturing sunlight are quickly overtopped. Over evolutionary
time, this has favored a proliferation of woody species capable of
establishing their place in the sun. The wet lowland tropics are
dominated by tropical rainforests—dense forests consisting of
species that are active all year round. The plants are either evergreen (that is, they always have leaves that are actively photosynthesizing) or leafless for only brief periods. The dominance
of trees influences all other aspects of the rainforest. The dense
shade means that plant cover beneath the trees in undisturbed
forests is sparse. But there are species that have adapted to these
conditions and are capable of growing in low light. The seedlings
and saplings of some trees are shade tolerant, meaning they can
maintain a positive energy balance despite the limited light, and
these are often the most abundant plants in the ground layer.
thought that Humboldt’s enthusiasm for
learning about new lands encouraged Jefferson’s own great scheme for the exploration
of the western United States. Thus, it is fitting
that Humboldt’s name is commemorated
in the names of several counties, mountain
ranges, and rivers in the American West.
Another class of plants—the epiphytes (meaning “on
plants”)—find light by growing on the trunks and branches of
the trees. Algae, mosses, and lichens grow as epiphytes in all climatic regions. The wet tropics have these in abundance, but also
a host of epiphytic species from the higher plant groups (Figure
32–6). Epiphytes survive without soil by having special adaptations to capture and retain moisture from the frequent rains and
by acquiring nutrients from animal droppings, plant litter, dust,
and dead insects. Epiphytes have evolved in many plant families,
but two well-known groups are the orchids and bromeliads (pineapple is a bromeliad); there are also many epiphytic ferns. Lianas, or woody vines also known as climbers, represent another
strategy for exploiting trees for support. Lianas are rooted in the
soil but grow up along the trunks of trees into the canopy. Some
lianas start as epiphytes and later drop stems to the soil, where
they take root. The “strangler figs” (Ficus spp., see page 558) are
a special case. They start as epiphytes, extend roots downward
into the soil, and eventually envelope and kill their host trees,
after which the strangler assumes an independent existence as a
canopy tree.
Because most edible leafy biomass in rainforests is in the
treetops, large herbivores and the predators that feed on them are
sparse on the forest floor. This, along with the favorable climate,
Rainforests 3 2 – 9
32–6 Epiphytes By collecting and storing
water and nutrients from the surrounding air,
rain, and dust, epiphytes create little patches
of soil from accumulated debris. Bromeliads,
seen here as leafy tufts growing along the tree
branches, are among the most common of
the epiphytes. The leaves of many bromeliads
merge at their bases to form watertight tanks
that, in the larger species, can hold as much
as 45 liters of rainwater. These pools of water
are microcosms of bacteria, protozoa, larvae,
insects, and insect-eaters. Many rainforest
mosquitoes breed exclusively in bromeliad
tanks. The bromeliads absorb water from their
built-in reservoirs and are also supplied with
nutrients from the debris.
explains the abundance of arboreal (tree-dwelling) animals,
including our primate relatives, such as the howler monkeys of
the Americas and the gibbon species of Asia, as well as an abundance of birds (parrots in the Americas and hornbills in Africa).
The mobility of primates and birds allows them to find the best
patches of foods in the treetop environments—fruits, insects, and
the more easily digestible new growth. It also makes them important dispersers of the seeds of rainforest plants.
Ecologists are especially interested in the numbers of species that occupy given amounts of space. If the metric is simply the total number in an area, the term “species richness” is
employed. Species diversity can be measured by species richness,
but to ecologists, “diversity” is a more general term that includes
consideration of the distribution of abundance among species.
For this general survey, we equate diversity with richness. By any
definition, though, tropical rainforests reach extraordinary levels
of plant species diversity. A typical hectare (2.47 acres, or about
2.5 American football fields) of boreal coniferous forest has
1 to 4 tree species, and a rich temperate deciduous forest, 10 to
20 species, but tropical rainforests commonly have 100 species,
and exceptionally rich forests have hundreds more. Epiphytes,
lianas, and the herbaceous plants of the forest floor provide additional species. This diversity of plant species supports a commensurate diversity of arthropods. As with all generalizations in
ecology, however, there are exceptions, and some forests of the
wet tropics are dominated by one or a few species of trees over
large areas.
Ecologists are still searching for a full explanation of why
tropical rainforests have such high diversity relative to other
biomes. Both temperature and moisture conditions are probably
important. The influence of warm temperatures is illustrated by
the fact that diversity is relatively higher not only in the wet tropics but also in the dry tropics, whereas it is universally much
lower in the polar regions. The situation with respect to moisture
is not quite so simple. The moist tropics are species-rich, and the
driest deserts are generally species-poor, but semiarid regions,
such as the shrub-dominated fynbos region of South Africa, have
quite high plant biodiversity.
Species diversity ultimately is dependent on the process of
evolution, and we expect diversity to be higher where there has
been more time and more favorable conditions for speciation—
so time for evolution may also be a controlling factor. Another
idea focuses on trophic interactions, especially parasitism (in the
general sense), as a source for selective pressure favoring the origin and persistence of species. The continuously warm and moist
conditions of the tropics allow fungi, bacteria, and herbivorous
insects to thrive and so to potentially act as selective agents in
evolution. The need to fend off evolving parasites and predators
may favor genetic diversity, which in turn may favor speciation.
Like all natural systems, rainforests are dynamic and constantly changing. Because of the abundant rainfall, natural fires
are rare. Rainforest trees have evolved to resist wind damage by
a variety of mechanisms, including decay-resistant wood and buttressing (Figure 32–7). But eventually, what goes up must come
down, and tree fall is a main natural disturbance in rainforests.
Tropical rainforests that lie within the reach of hurricanes or
typhoons can suffer widespread canopy loss, but such catastrophes are not essential for tree fall. Individual tree falls are also
agents of change. A single falling tree often brings down other
trees around it, by direct impact or because the trees are connected by lianas. Light can then penetrate to the forest floor, and
a strong pulse of growth by herbs, shrubs, seedling trees, and
resprouted trees follows. If the gap is large enough, sun-requiring
species, dispersed to this spot, can establish themselves before the
gap closes.
Rainforests Are Rapidly Being Destroyed
Tropical rainforests have already lost half of their original area,
and the loss continues. It is estimated that at current rates, only
about 5 percent of the Earth’s area originally covered by tropical rainforests will survive by the middle of this century. Since
32–10
C h a p t e r 3 2 Global Ecology
(a)
(c)
32–7 Tropical rainforest (a) The
interior of a rainforest in Costa Rica. The
broad-leaved plants with red flowers
are Heliconia irrasa. A female green
anole (Norops biporcatus) is seen in the
foreground. (b) The diversity of the trees
in the forest, which may reach several
hundred species per hectare, is revealed
when individual trees burst into bloom,
such as these trees in the coastal rainforest
of Brazil. (c) A buttressed tree in the
rainforest of Ecuador. Note the woody
vines known as lianas on the trunk.
(b)
the tropics contain half of the world’s species, the loss could have
dire consequences—certainly for global biodiversity. The greatest
threats are exploitative logging and the subsequent conversion of
forest to cropland and open pastures. Agricultural clearing makes
landscapes susceptible to fire to a degree that is historically
unprecedented, as was tragically demonstrated by the destructive
fires of 1997–1998 in the Kalimantan region of Indonesia, where
over 5 million hectares of forest and former forest burned.
The conversion of tropical ecosystems to cropland would be
more defensible if it always resulted in highly productive agricultural lands. But experience shows otherwise (Figure 32–8).
Tropical soils often prove to be fragile. They can lose fertility
rapidly when cleared, because a high proportion of their nutrient
capital is tied up in the vegetation and proportionately less in the
strongly weathered mineral portion of the soil. Organic matter,
important for soil structure and retention of nutrients, is confined
mostly to a shallow layer easily depleted during the clearing of
trees. Addition of fertilizer may not help much, because tropical
soils often fix phosphorus into insoluble forms.
Deciduous Tropical Forests
Only some parts of the tropics receive reliable year-round rainfall. In regions with a prolonged dry season, tropical rainforest
gives way to other, more drought-tolerant forest types. These
forests can be grouped under the general term of tropical seasonal forests. For example, in central and southern Africa, the
Indian subcontinent, and Southeast Asia, there are extensive
Savannas 3 2 – 11
32–8 Erosion Of the native vegetation
that once covered Madagascar, an island
in the Indian Ocean, 420 kilometers east
of the African mainland, only 10 percent
remains, and that is fast disappearing. On
Madagascar, as in other tropical areas,
the soils exposed by clearing vegetation
are often not suitable for sustainable
agriculture. The abundant rains rapidly
wash the topsoil off increasingly barren
landscapes, choking the rivers with silt,
as seen here, and sometimes causing
floods and landslides. The sea around
Madagascar is often rust-red from the loss
of the island’s weathered red soil, and it is
difficult to produce enough food to sustain
the people of the world’s fourth-poorest
country.
areas of monsoon forest, and similar climates and forests are
found in northern Yucatán Peninsula in Mexico, in northern
Colombia and Venezuela, in Ecuador, and in eastern Brazil (Figure 32–2). These climates produce enough rain to permit vigorous tree growth resulting in dense and productive forests, but
the dominant trees are deciduous, losing their leaves during the
dry season. Elsewhere in areas of the tropics with seasonal rainfall, there are other types of dry forests, such as the short-tree
forests of tropical Mexico and Central America and the biologically distinct dry forests of Madagascar. Tropical mixed forests, in which both evergreen trees and shrubs are present, occur
locally in eastern and southern Brazil and northern Australia
(Figure 32–2).
Although tropical rainforests tend to dominate public attention, the seasonally dry forests of the tropics also contain very
high levels of biodiversity. Like the rainforests, dry forests are
also being rapidly lost to exploitative land use or abused by
unsustainable forest harvest.
Savannas
With further decreases in rainfall, a point is reached at which
moisture is not sufficient to sustain a continuous tree canopy.
Trees and shrubs are scattered individually or grow in groups
in a grassland setting, and in places, woody plants nearly disappear. Such areas are called savannas, a term derived from the
native languages of the Caribbean. The precise definition varies
regionally. In South America, for example, savannas include
tropical grasslands that are virtually free of trees, whereas in
other places the term is restricted to grasslands with a significant
woody component (Figure 32–2).
Savannas of all kinds have annual rainfall in the range of 90
to 150 centimeters. In the tropics, savanna trees are broad-leaved
deciduous and evergreen species, which may grow singly or
in small groves. Other savanna types have a high proportion of
shrubs (Figure 32–9). The cerrado vegetation of Brazil belongs to
the savanna biome. It varies from open grassland to almost closed
forest.
In savannas, by definition, herbaceous plants, especially
grasses, dominate. Bulbous plants can be abundant. Because of
the dense cover of perennial herbs made possible by the relatively
high seasonal rainfall, there are few annual herbs. Trees are well
branched, but they are seldom more than 15 meters tall. Many
trees, such as the acacias, are protected by stout spines, which
discourage grazing (see Figure 31–9a). Leaves are generally
smaller than those of the evergreen trees of the rainforest, and
they are better able to regulate their water loss.
The prolonged dry season, combined with the presence of
highly flammable dry herbaceous growth, means that savannas
are prone to burning. In general, fire is more destructive to woody
plants than to herbaceous species. Unless protected by thick bark,
a woody plant may lose all its investment in aboveground structures and will have to regrow years of accumulated biomass. If
fires recur before recovery is complete, the tree may be killed or
32–12
C ha p t e r 3 2 Global Ecology
32–9 Savanna A savanna in East Africa,
with giraffes surrounded by a herd of
impala. The transitional nature of this
biome, relative to the characteristic
vegetation of the tropical rainforest
biome and the desert biome, is evident
in the grasses, shrubs, and short trees
(acacias) seen here.
reduced to a shrubby form, often arising from an enlarged base,
or burl. But a herbaceous plant that is dormant at the time of a
fire, and protected by having its live tissue below ground, will
usually not suffer significant damage. Thus if fires occur regularly, they can gradually push back closed forests, opening the
ground to full sun and encouraging the dominance of grasses and
other herbaceous species. Current opinion is that natural fires
must have been common enough to cause the evolution of fireresilient plants before burning by humans became so prevalent.
This is not as certain in Africa, where fire-promoting hominoids
have existed for periods long enough for evolution to act, but
certainly applies in the Americas, where humans, in evolutionary terms, arrived very recently. There is little doubt that many
savanna areas have been modified to some extent, possibly even
created, by humans.
At their poleward margins, tropical savannas and related
plant communities grade into deserts, because of the low precipitation enforced by the global circulation (Figure 32–3). This transition is more or less continuous and occurs over a broad region
in Africa, but it is more complex in the New World and in Asia.
One such transitional community is the widespread juniper (Juniperus spp.) woodlands of western North America (Figure 32–10).
These occur in places that are usually too dry to support grasslands but wetter and cooler than desert areas. In many places
in the western United States, juniper, often found with pinyon
pine, grows in areas adjacent to deserts but at higher elevations.
Studies of pollen and macrofossils preserved in pack-rat middens show that vegetation types that seem to be permanent features of the landscape actually are dynamic over time. During
the Pleistocene epoch, at the times of maximum expansion of the
(a)
(b)
32–10 Juniper woodlands (a) Juniper (Juniperus osteosperma) savanna in the Great Basin near Wellington,
Utah. (b) Cold winters are characteristic of the pinyon-juniper savannas and woodlands of the Great Basin, as
shown here south of Moab, Utah.
Deserts 3 2 – 13
continental glaciers, juniper woodland communities moved down
into the lowland areas that today support juniper-free deserts.
Where temperatures are above freezing for much or all of the
year, and where precipitation is so meager and unreliable that it
can no longer support a continuous cover of perennial vegetation,
we find the desert biome. All the great deserts of the world are
located outside the equatorial zone, in the zones of atmospheric
high pressure that flank the tropics at about 30° north and 30°
south latitudes. They extend poleward in the interior of the large
continents (Figure 32–2). Extensive deserts are located in North
Africa and in the southern part of the African continent. Other
deserts occur in the Near East, in southeast Mongolia and northern China, in western North America and western and southern
South America, and in Australia. The Sahara, which extends all
the way from the Atlantic coast of Africa to the Arabian peninsula, is the largest desert in the world. Seventy percent of the
entire continent of Australia is covered in semiarid or arid areas.
Less than 5 percent of North America is desert, and much of that
is not extreme (Figure 32–11).
(a)
(b)
(c)
(d)
Deserts
32–11 Desert Some representative plants of the principal deserts of North America. (a) The Sonoran
Desert stretches from southern California to western Arizona and south into Mexico. A dominant plant is the
giant saguaro cactus (Carnegiea gigantea), which is often as much as 15 meters high, with a wide-spreading
network of shallow roots. Water is stored in a thickened stem, which expands, accordionlike, after a rainfall.
(b) To the east of the Sonoran is the Chihuahuan Desert; one of its principal plants is the agave known as
the “shin dagger” (Agave lechuguilla), a monocot. (c) North of the Sonoran is the Mojave Desert, with its
characteristic plant, the Joshua tree (Yucca brevifolia). This plant was named by early Mormon colonists, who
thought that its form resembled that of a bearded patriarch gesticulating in prayer. The Mojave contains
Death Valley, the lowest point on the continent (90 meters below sea level), only 130 kilometers from Mount
Whitney, with an elevation of more than 4000 meters. (d) The Mojave blends into the Great Basin Desert,
a cold desert bounded by the Sierra Nevada to the west and the Rockies to the east. It is the largest and
bleakest of the American deserts. The dominant plant is sagebrush (Artemisia tridentata), shown here with
the snow-covered Sierra Nevada in the background.
32–14
C ha p t e r 3 2 Global Ecology
The more severe deserts receive less than 20 centimeters
of rainfall per year (see Figure 31–6). In the Atacama Desert of
coastal Peru and northern Chile, the average rainfall is less than
2 centimeters per year. Averages, however, do not tell the whole
story. As average rainfall decreases, the variability from year
to year increases. In a humid region, a severe drought would be
50 percent less rain than average; in a desert, a drought is often
no precipitation at all. The plants and animals of arid regions
must be able to tolerate these periods of extreme dryness and
to exploit the good years when they occur. This can favor some
bizarre-appearing plant types, notable examples being Welwitschia (see Figure 18–41) found in the extreme coastal desert
of Namibia in southern Africa and the cirio (Fouquieria columnaris) of Baja California (Figure 32–12).
The desert regions of western North America illustrate the
effect of larger-scale climatic variation. The hot deserts occur
in the lower latitudes. The deserts of Arizona, which are of this
type, are characterized by two peaks in rainfall, both variable
with respect to exact timing and amounts. This pattern favors succulents that can store water from the infrequent and often scant
rains. Temperatures can reach extremes, because the skies are
32–12 Surviving extreme dryness With leaves that emerge
sporadically, depending on the presence of rain or coastal fog, the
cirio (Fouquieria columnaris), also known as the boojum tree, is
able to survive extensive periods of little or no moisture. “Boojum”
comes from a nonsense poem by Lewis Carroll.
usually clear, and little heat is lost to evaporation, because of dry
soils, low plant cover, and limited transpiration. Summer temperatures of more than 36°C are common. The same clear-sky
conditions that make deserts hot in the day cause them to cool
rapidly at night. The heat stored during the day is lost through reradiation, and temperatures drop sharply as soon as the sun sets.
As we move north from Arizona, the pattern in deserts shifts
toward precipitation falling primarily in the winter, some of it
in the form of snow. Such cold deserts are found, for example,
in the Great Basin Desert of western North America, which lies
between the Sierra Nevada–Cascade Mountain system and the
Rocky Mountains (Figure 32–11d). In this region, winters can
be bitterly cold. Succulent species are much less common, and
small-leaved shrubs, such as the widespread sagebrush Artemisia
tridentata, predominate. Moving further poleward, we reach
areas of low precipitation in the Arctic and Antarctic regions,
where the extreme cold combines with limited moisture to
restrict plant life primarily to hardy lichens and mosses.
Desert Plants Are Adapted to Low Precipitation
and Extremes of Temperature
High variability in moisture conditions means that desert plants
must be able to either survive the dry season or dry periods in a
dormant condition or have some way of averaging out variable
moisture conditions. One solution is the annual-growth strategy, surviving the dry times as seeds that germinate when rains
soak the surface, and flowering and setting a new crop of seeds
before the window of moist conditions closes. Many species
have been able to adopt this strategy, and there are proportionately more annual species in the deserts and semiarid regions of
the world than in other biomes. For perennial plants, dormancy,
water storage, access to deeper water, or a high level of resistance to drought stress are the only options. Grasses as a group
have drought-tolerating features and are present at some level in
all but the most extreme deserts. A typical perennial desert grass
has a tussock growth form—a dense cluster of stems. With the
onset of drought, the leaves die back, and the regenerative buds
are protected in the mass of dead stems and leaves. Other herbaceous plants have deeply buried bulbs that break their dormancy
when stimulated by rain.
The amazing capacity of plants to adapt to desert conditions
is exemplified by the grayish white wooly Arizona honeysweet
(Tidestromia oblongifolia), a C4 (pages 140 to 145) perennial
sub-shrub—that is, a plant with a shrubby growth form but with
woody tissues only at the base of the plant—common in the
Death Valley region of California and Nevada. The maximum
photosynthetic rate for this species is achieved at temperatures
between 45° and 50°C (around 120°F) in full sunlight in midsummer—temperatures that would be damaging or even fatal to
plants of regions with higher moisture and cooler temperatures.
Succulents are a distinctive class of water-storing desert
perennials. The capacity to store water is found in many plant
families, but two groups are notable: the cacti of the New World
(family Cactaceae), essentially all of which are succulents, and
the succulent euphorbs of the Old World (Euphorbiaceae). Members of these distantly related families provide some of the best
examples of convergent evolution (see the essay “Convergent
Grasslands 3 2 – 15
32–13 Creosote bushes One of the most
characteristic plants of the Mojave, Sonoran,
and Chihuahuan deserts of North America is
the creosote bush (Larrea divaricata), which
has small, leathery, water-conserving leaves.
Creosote bushes in the Mojave Desert may
form circular or elliptical clones, because
of new stem production at the periphery
of stem crowns and the segmentation and
death of older stem segments, resulting in
a ring of satellite bushes around a central
bare area. The latter usually accumulates a
mound of sand, which may reach a depth of
about half a meter. Some clones may attain
extreme ages: the King Clone, shown here,
is estimated to be nearly 12,000 years old.
Such ancient clones started from seeds that
germinated near the end of the last glacial
expansion.
Evolution” on page 239). Succulents have shallow root systems
that are designed to absorb water while it is available and then
use it conservatively. To accommodate this water, succulents
have a morphology that allows them to change volume without
damage, a feature especially obvious in the accordionlike structure of many cacti. CAM (crassulacean acid metabolism—named
for the Crassulaceae, a family with many succulent species;
pages 145 to 147) photosynthesis provides another means of conserving the stored water. CAM plants reverse the usual pattern
and open their stomata at night, then close them during the day,
greatly reducing their loss of water (see the essay “How Does a
Cactus Function?”). The success of the succulent life history is
evidenced by the fact that succulents often are among the tallest
and most robust plants of deserts—an example being the iconic
saguaro cactus (Carnegiea gigantea) of the Sonoran Desert of
the southwestern United States (Figure 32–11a).
Another large group of perennial shrubs do not store water
but have the ability to average out variation in rainfall by tapping into water reserves with their deep and extensive root systems. The success of this strategy is exemplified by the creosote
bush (Larrea spp.), a genus of several species dominant over
vast areas of New World deserts (Figure 32–13). Because deep
water is accumulated by slow downward percolation, it varies
much less than surface water over time and is available during
dry seasons and even through several dry years. Tapping deep
water would not work, however, if the plants were not able to use
water efficiently. Creosote bushes produce small, leathery leaves
with relatively few stomata and have stems resistant to the loss
of water conductivity that is caused by embolisms (air pockets
that block conducting elements). These characteristics enable
close control of water loss and allow the plant to resist wilting
when under extreme moisture stress. C4 photosynthesis is likewise more common among the plants of deserts and other seasonally dry, warm habitats than it is elsewhere. The plants also
have the capacity to drop leaves and even entire branches in dry
periods so that they can maintain their water balance. Then, when
better conditions return, they can regenerate their canopies. Other
woody plants, such as the various species of palo verde (e.g., Parkinsonia spp.), have green stems rich in chlorophyll and so are
able to photosynthesize even when leafless.
Grasslands
Grasslands occur where the amount of rainfall is less than is
needed to support vigorous tree growth but great enough to allow
grasses and other herbaceous plants to produce a more or less
uniform cover. They are found from the tropics to the edges of
the boreal regions and from sea level to high mountain meadows.
They grade into savanna (as noted above, grasslands in tropical
areas are sometimes considered a type of savanna) and into desert. They occur as patches in woodland and forest when historical factors (such as fire) or soil factors (shallow dry soils) reduce
tree growth. In the central and eastern United States, such grassy
patches are often called glades, and they are typically dominated
by prairie plants (Figure 32–14). Grasslands are most extensive,
however, in the middle latitudes, where they coincide with the
strongly seasonal climates of the temperate zones (Figure 32–2).
Grasses regenerate at the beginning of each growing season
from buds at or below ground level and have many fine roots
designed to exploit the resources of the surface soil. Over long
periods of time, grasslands, with the assistance of a complement
of burrowing animals, tend to build soils with deep organic layers
(see Figures 29–5, 29–6). Such soils are ideal for agriculture, and
vast areas of grassland have been converted to cropland. In areas
too dry for agriculture and without irrigation, grasslands have
value as grazing lands. Although the growth of grassland plants
is seasonal, there is little room for the development of annual
herbs, and these are mostly limited to disturbed areas, such
32–16
C ha p t e r 3 2 Global Ecology
32–14 Glade Coreopsis dominating a glade in temperate
deciduous forest near St. Louis, Missouri. Such openings in
the forest usually occur in areas of shallow soil and are often
dominated by prairie plants that form continuous stands beyond
the borders of the forest.
as prairie-dog towns or badger diggings or areas disturbed by
humans, such as near buildings and along roadsides and railroads.
The major variations in grasslands relate to precipitation.
Grasslands receiving the highest precipitation are dominated by
tall grasses in closely spaced clumps or dense sods. With decreasing precipitation, the stature of the plants decreases and the
degree of coverage declines. In the driest grasslands the plants
occur in widely spaced clumps, with sparse vegetation between
them. The major gradients are exemplified in North America.
Moving from west to east there is a transition from desertlike,
western shortgrass prairie (the Great Plains), through intermediate midgrass prairie, to the moister and richer tallgrass prairie
(now the Corn Belt) that gives way, in a complicated pattern, to
the eastern temperate deciduous forest (Figure 32–15).
Grasslands become progressively drier with increasing distance from the Atlantic Ocean and the Gulf of Mexico, which
are the major sources of moisture-bearing winds in the eastern
half of the North American continent. Farther north, grasslands
become more moist again as evaporation decreases at relatively
cool temperatures.
Because most grasslands have more or less continuous cover
and die back for part of the year, they carry fire readily. Where
woody plants and grasses coexist, fires carried in grasses can
“crown” (that is, spread fire within the tree canopies) or kill some
or all of the meristematic tissues at the base of trees and shrubs.
Though fires have always occurred naturally, it is thought that the
purposeful burning of grasslands, for example, by Native Americans, may have increased the amount of grassland relative to forest. Those who manage natural grasslands today commonly use
fire as an essential tool (see Figure 31–16b).
Our emphasis here has been on plants, but all of the great
grasslands of the world were once inhabited by herds of grazing mammals and associated large predators. Though there is no
doubt that the grazers depend on an abundance of grass, there is
debate about the extent to which grazing animals are necessary
for maintaining the health of grassland vegetation. Some have
argued that grasses have evolved to require pruning by grazers to
(a)
32–15 Grasslands The grasslands of North America include large regions of shortgrass and tallgrass
prairie. (a) A female bison nursing her calf in the shortgrass prairie of Custer State Park, South Dakota.
(b) A June day on a tallgrass prairie in North Dakota. The cottonwood grove by the prairie creek is
characteristic of this biome.
(b)
Grasslands 3 2 – 17
H o w D o e s a C act u s F u nct i o n ?
The barrel cactus, Ferocactus acanthodes,
which grows along the northwestern edge
of the Sonoran Desert in southern California,
gets its name from its barrel-like shape. It looks
as if its stem has been folded like an accordion,
and it is covered with spines like a porcupine.
Why should this desert plant present such an
unusual and formidable appearance?
When the barrel cactus is full of water, the
folds swell and are barely visible, but when
the plant dries, the folds are deep and the
stem can contract without crushing the cells.
This ridge-and-valley folding of the stem has
other advantages, too. Deep inside the valleys between the folds are the stomata. The
spines help to break up the wind currents,
and thus the valleys serve as protected retreats where dry desert winds cannot easily reach to carry moist air away from the
vicinity of the stomatal chambers. These
formidable spines also help to protect the
cactus from the rodents and birds that are in
constant search of water, even going so far
as to steal it from a succulent stem. A study
conducted by Park Nobel, of the University
of California, Los Angeles, found that barrel
cacti stored enough water in their succulent
stems to permit them to open their stomata
for about 40 days after the soil became too
dry to furnish them with any additional water. Under such conditions, many of the fine
roots are sloughed off, to prevent water loss
to the soil. After seven months of drought,
stomatal activity ceased, and the osmotic
potential of the stem was more than double
the value it had been during wet periods,
despite the ability of the stem to fold and
shrink. When rain finally returned, the shal-
low root systems (with a mean depth of only
about 8 centimeters) took in water so rapidly
that the stomata were fully functional again
within 24 hours of rainfall.
But these are not the only mechanisms
the barrel cactus uses for conserving water.
Like many other desert plants, the barrel
cactus exhibits crassulacean acid metabolism (CAM). It opens its stomata only at night
and so undergoes gas exchange with cooler
air, which can hold less water than warmer
air. Consequently, the plant loses less water
to the atmosphere through transpiration.
Its ratio of mass of water transpired to mass
of CO2 fixed is only about 70:1 for the entire
year. This is considerably lower than for a
typical C3 plant, which requires larger quantities of water to fix an equivalent amount of
carbon but has a higher maximum rate of
photosynthesis.
Because seedlings of the barrel cactus, unlike their parents, cannot tolerate extremely
high temperatures and prolonged drought,
they survive only in certain years and in protected microhabitats. By the age of 26 years,
these plants have usually grown to only about
34 centimeters tall, adding about 10 percent
to the mass of their stems each year.
Barrel cactus in a hydrated state (left), and in a dehydrated state (right).
maintain a healthy photosynthesizing canopy. Whether or not this
is correct, it is undeniable that grasses, grazers, and their predators have formed sustainable systems.
Humans have disrupted natural systems, however. In the
American West, the main natural grazer, the bison, was hunted
to the brink of extinction and survives today only in refuges. The
role of the bison in those grasslands that were not converted to
row crops was taken over by domestic animals—mostly cattle
and sheep. But domestic animals are not limited by wild predators, and since humans tend to maximize the size of their herds,
overgrazing has been historically common. This is thought to
account for the drastic depletion of the grasslands that formerly
stretched from southern Arizona to western Texas. Over the
course of the last century and half, these landscapes were converted to deserts or shrublands with sparse grass. The failure of
farmers and ranchers to manage grasslands properly also contributed to the “dust bowl” disasters of the central United States in
the 1930s (Figure 32–16). In extreme cases, the combination of
grazers (mostly eating grass) and browsers (mostly eating woody
vegetation) can turn lush areas into deserts of spiny and unpalatable plants. But responsible managers of grazing have learned
from these past mistakes and have developed methods that make
32–18
C ha p t e r 3 2 Global Ecology
32–16 Dust bowl The prairie soils were once
so bound together with the roots of grasses
that they could not be cultivated until adequate
plows were developed. But once the plants
were removed by overgrazing or careless
cultivation, prairie soils rapidly deteriorated and
were blown away by the wind. This photograph
of a badly eroded farmyard, taken in Oklahoma
in 1937, vividly recalls the “dust bowl”
conditions that led many thousands of people
to migrate away from the central United States.
John Steinbeck’s novel The Grapes of Wrath was
based on the experiences of these migrants.
sustainable grazing possible. With proper management, some lost
grasslands, so it is hoped, can be restored.
Temperate Deciduous Forests
As we move poleward from the tropics, seasonality increases, and
eventually, at about 35° latitude, we encounter climates experiencing long periods of temperatures below freezing. Even though
the climates in these regions range between tropical heat and
moisture in the summer and Arctic cold in the winter, it is traditional to describe them as “temperate” and the deciduous forests found there as temperate deciduous forests. These forests
are best developed in areas with warm summers and cold winters
(Figure 32–17). Annual precipitation generally ranges from about
75 to 250 centimeters and is either distributed evenly throughout
the year or concentrated somewhat during the summer months.
Temperate forests occur in both hemispheres, but because of the
limited land area in the appropriate southern latitudinal bands,
they occupy correspondingly small areas in the Southern Hemisphere (Figure 32–2).
The tree form is favored in these regions, for the same reasons it is favored in the rainforests, though the kinds of trees and
the structure and function of the forests are quite different. Leaf
fall is strongly seasonal, with most of the trees remaining leafless through the winter. The deciduous trait can be explained
by energetics. Relatively ample nutrients and moisture available
(a)
32–17 Temperate deciduous forest Representative plants
of the temperate deciduous forests of North America. (a) A
beech and maple forest in Michigan, photographed in the
spring. The forest floor is carpeted with large-flowered trillium
(Trillium grandiflorum). (b) In the fall, as seen here in a forest in
the southern Appalachian mountains, the leaves of maples and
eastern sourwoods (Oxydendrum) turn a beautiful scarlet.
(b)
Temperate Deciduous Forests 3 2 – 19
32–18 Annual growth cycle The four seasons in a temperate deciduous forest in Illinois. The trees
leaf out early in spring and begin to manufacture food; they lose their leaves in autumn and enter
an essentially dormant state, in which they pass the unfavorable growing conditions of winter.
Many herbs grow under the trees (see Figure 32–17), and some of them flower very early in spring,
before the tree leaves reach full size and shade the forest floor. In spring, most of the trees produce
abundant pollen, which is carried by the wind.
throughout the summer assure high productivity, so that forming a new leaf produces larger gains and involves fewer risks
than maintaining a more energetically expensive evergreen leaf
through the rigors of the cold season. One of these risks is the
lack of cold-season soil moisture because of frozen soil, a situation some describe as physiological drought.
The annual cycle of growth is a defining feature of temperate deciduous forests (Figure 32–18). In winter, when the
trees are leafless, their metabolic activity is greatly reduced.
As temperatures warm, stored sugars are mobilized (the “sap
rises”) and dormant buds begin to develop. It takes time for the
new branches and leaves to expand fully, and this results in a
short period when the forest floor receives substantial direct
sunshine with temperatures above freezing. A variety of herbaceous plants, many with showy flowers, have evolved to exploit
this situation (Figure 32–17a). Some (the spring ephemerals)
have leaves that emerge seemingly overnight from bulbs or rhizomes. They complete most of their growth and reproduction
before the trees are fully leafed out, then die back after the canopy closes. Others better adapted to reduced light (for example,
early and late summer species and evergreen species) emerge
more slowly and sustain photosynthetic activity during the shady
conditions of summer. These species generally have leaves that
are broader but thinner in cross section than those active only in
spring, and they usually have smaller storage organs relative to
their size.
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C ha p t e r 3 2 Global Ecology
Most of the species that ripen their seeds in spring are dispersed by ants (page 495), which are active when few other
dispersers are present. In contrast, most of the fall-ripening species are dispersed by birds, during a season that coincides with
the major bird migrations toward the equator. Very few annual
plants occur in deciduous forests, and a smaller number of these
behave as spring ephemerals. The cooler temperatures of these
forests constrain the early growth that is crucial if an annual is
to achieve reproductive size, a problem that spring ephemerals
solve by having energy-producing stores (roots, bulbs, and rhizomes) below ground.
A striking feature of the temperate deciduous forest is the
sharing of genera—and, in some cases, even species—among
three main regions in the Northern Hemisphere. Study of the fossil record shows that in the earlier Cenozoic era, these genera
formed a band of deciduous forest across the Northern Hemisphere. Most of the deciduous trees and associated herbs were
eliminated in western North America during the latter half of the
Cenozoic, as the amount of summer rainfall was greatly reduced.
This history explains why the herbaceous flora of the deciduous
forests of China and Japan taxonomically resemble that of eastern
North America more closely than either resembles that of western
North America. But relics of the deciduous forests are still to be
found in the western United States—for example, redbud (Cercis
occidentalis) and big-leaf maple (Acer macrophyllum).
Temperate Mixed and Coniferous Forests
Although deciduous forests are the defining forest type in vast
areas of the midlatitudes, there are also extensive areas with
conifers, occasionally as the dominants, more often mixed with
so-called broad-leaved species. We consider these as temperate mixed and coniferous forests. In keeping with the fact that,
evolutionarily speaking, conifers predate the broad-leaved angiosperms, they tend to be most abundant where conditions are not
the most favorable for tree growth—such as poorly drained sites,
nutrient-deficient sites, and sites with shallow soil.
Bordering the deciduous forests on the north (as noted
above, the temperate forest is limited in the Southern Hemisphere) are forests in which conifers are prominent. Such temperate mixed forests (Figure 32–19) are characteristic of the
Great Lakes–Saint Lawrence River region, eastern Europe, the
northern and eastern border regions of Manchuria (in northeast
China) and adjacent Siberia, eastern Korea, and northern Japan
(Figure 32–2). They can be considered the intermediate condition between temperate deciduous forests to the south and the
conifer-dominated taiga to the north. Temperate mixed forests
occur in areas with colder winters and more reliable snow cover
than are found in areas of temperate deciduous forest. The conifer element dominates on nutrient-deficient soils such as glacial
outwash sands.
There are also mixed forests on the southern edge of the
temperate forest—the subtropical mixed forest (Figure 32–20).
These forests occur in a complex mosaic of riparian (riverrelated) forest and swamp, with deciduous and broad-leaved
evergreen forests on richer soils. Pines are especially prominent
in these mixed forests, and in the southeastern United States,
loblolly (Pinus taeda), slash (Pinus elliottii), and longleaf
pines (Pinus palustris) are the common species. These pines
32–19 Temperate mixed forest In this forest in southern Ontario,
the evergreen conifers appear dark green and the deciduous trees
are showing their fall colors.
are commercially important because of their fast growth and
straight trunks. Although reduced soil fertility is a major explanation for the presence of conifer species, the cycle of disturbance also plays a role.
32–20 Subtropical mixed forest Slash pine (Pinus elliottii) is one
of the widespread evergreen conifers growing in the subtropical
mixed forests of the southeastern United States. Conifers tend
to dominate on soils that are either nutrient-poor or seasonally
flooded, or both.
Longleaf pine has a distinctive life history related to fire. As
a seedling, it exists in a grasslike form, with its long needles protecting its stem and growing point from fire. After some years,
the seedling bolts, rapidly growing through the most vulnerable
stages to a size that can survive fire. When fire is excluded from
historical longleaf stands, they are invaded by broad-leaved species. Another fire-adapted species is the Florida sand pine (Pinus
clausa), which grows in dense stands on nutrient-poor sandy
soils. Unlike the longleaf pine, sand pine has serotinous (“late
or delayed opening”) cones that are opened by the heat of fire.
Because fire also kills most trees, the stands are reestablished by
seed dispersed from the fire-opened cones.
Cypress swamps are another conifer-dominated system characteristic of the subtropical mixed forest zone. Spanish moss–
draped bald cypress (Taxodium distichum; “bald” because it
is usually deciduous) is an iconic species of the swamps of the
southeastern United States (Figure 32–21). It is most abundant in
intermittently flooded sites, where it can reach ages in excess of
200 years and diameters of 2 meters or more.
The current vegetation of western North America, the
forested part of which has an abundance of conifer species, is
an intricate mosaic complicated by the presence of mountain
Mediterranean Scrub 3 2 – 21
32–22 Alpine tundra As seen here on the Olympic Peninsula in
Washington State, alpine tundra is comparable in many respects to
the Arctic tundra found hundreds of miles to the north. In this area,
however, forested slopes are found within a hundred meters or so
of the alpine meadows.
ranges and their sequence of elevation-related changes in vegetation. Species that have expanded from the south formed, at
lower elevations, grassland and shrubland communities along
with savanna-like areas of evergreen and some deciduous oaks.
Occurring at the highest elevations in these regions are the
open herbaceous-dominated communities called alpine tundra
(Figure 32–22), which in North America are intermixed with
mountain forests.
To the north, coniferous forests and mixed West-coast forests predominate, with such trees as the coast redwood (Sequoia
sempervirens; Figure 32–23), the big tree or giant sequoia
(Sequoiadendron giganteum), the Douglas fir (Pseudotsuga menziesii; Figure 32–24), and the sugar pine (Pinus lambertiana;
Figure 31–15b), all of which were much more widely distributed
in the past. Similar kinds of conifer-dominated vegetation are
found in areas with similar climate characteristics in Scandinavia,
central Europe, the Pyrenees, the Caucasus, the Urals, southern
Tibet, and the Himalayas northward to eastern Siberia. Vegetation types that resemble these species are also found in areas in
western South America, central New Guinea, southwestern
New Zealand, the southern Arabian peninsula, Ethiopia, and the
mountains of Central Africa (Figure 32–2).
Mediterranean Scrub
32–21 Stabilized trees The bald cypress (Taxodium distichum)
is a commercially valuable species that occurs in the low-lying,
intermittently flooded swamps of the southeastern United States.
The trees are often draped with Spanish moss (Tillandsia usneoides),
which is not a moss but an epiphyte in the Bromeliaceae
(pineapple family). The swollen bases and buttress roots stabilize
the trees in the soft, usually highly organic substrate.
Highly distinctive shrub communities have evolved from mixed
deciduous-evergreen forests in areas with Mediterranean climates characterized by cool, moist winters and hot, dry summers.
Such climates are found along the shores of the Mediterranean
Sea, along the western edge of California and southern Oregon,
in central Chile, on the southern coast of Africa, and along portions of the coast of southern and southwestern Australia (Figure
32–2). The plants in these areas have growing seasons concentrated in the cool, moist part of the year. The luxuriant growth of
32–22
C ha p t e r 3 2 Global Ecology
32–23 Mixed West-coast forest Redwoods (Sequoia sempervirens)
32–24 Pacific coast rainforest Douglas firs (Pseudotsuga
are a prominent feature of the mixed west-coast forests of
California. Watered by the frequent fogs of the region during the
dry summers and protected from freezing temperatures by their
proximity to the ocean, redwoods often form spectacular groves,
many of which, like the one shown here in Muir Woods near San
Francisco, are now protected in parks and reserves.
menziesii) growing on the Olympic Peninsula in Washington State.
Epiphytic mosses, liverworts, Selaginella, and lichens often grow
luxuriantly on the trees. The forests of this area of North America
experience heavy precipitation and are dominated by conifers.
They are the basis for a profitable lumber industry.
late winter and spring is followed by summer-fall drought, with
some species becoming dormant or shedding most of their leaves,
and others remaining evergreen and enduring the dry weather by
drawing on deeper moisture. Though there are patches of woodland and grassland, vast areas are covered by shrublands with a
continuous canopy, forming a kind of miniature forest. Shrublands of this kind are found in each of the widely dispersed Mediterranean climate regions and each has its own local name. In
California and elsewhere in western North America, chaparral
is the word for mostly evergreen and typically very dense shrublands. The equivalent vegetation around the Mediterranean Sea is
called maquis (Figure 32–25), in Chile it is known as matorral, in
South Africa fynbos, and in Australia mallee (kwongan in Western Australia). The quintessential growth form of the Mediterranean shrublands is the so-called sclerophyllous (“tough-leaved”)
evergreen, deep-rooted species with stiff leaves that resist wilting. Another characteristic form is summer-drought deciduous, or
semideciduous, with softer leaves, often with aromatic secondary
compounds.
Viewed from a distance, these different shrublands have a
similar appearance. Even close up, species of different regions
may appear superficially similar. But unlike the situation in the
northern temperate forest, the similarities arise mostly because of
convergent evolution. Each region has taxonomically unique species that have evolved toward morphological and physiological
similarity.
In all Mediterranean regions fire is a factor, though there
is reason to believe that it is not uniformly so. Evidence of fire
selection may be seen in the life histories. Most common—and
not so clearly fire-selected—is vigorous resprouting after fire,
often from an enlarged woody structure called a burl or lignotuber
(“woody tuber”). Other species, the obligate seeders, are killed by
fire but germinate from seeds that have accumulated in the soil
between fires. Another strategy is to store seed in long-persisting,
serotinous fruiting structures that release seeds in large numbers
only after fire. Fire-stimulated seed release is hard to explain
except for selection by fire, so the relative abundance of species
with this strategy suggests that fire selection has been historically significant. Since Australia and South Africa have the largest
number of such species, these regions also have probably experienced the most fire over evolutionary time. The California Mediterranean region, on the other hand, has many obligate seeding
species that build up seed banks in the soil, with the serotinous
trait found only in two genera of conifers (Cupressus and Pinus).
The Northernmost Forests—Taiga and Boreal Forest 3 2 – 23
(a)
(b)
32–25 Mediterranean-type vegetation Regions with Mediterranean-type climates are characterized by
hot, dry summers and mild winters, with strongly seasonal rainfall that occurs primarily in the cooler part of
the year. The vegetation is dominated by dense, low, mostly evergreen or summer-deciduous shrubs. There
is a striking superficial similarity in the vegetation, whether in California, Chile, Australia, South Africa, or the
Mediterranean region, as seen here for (a) chaparral in southern California and (b) similar vegetation, called
maquis, on the Greek island of Corfu. Each region, however, has its unique set of species.
The Mediterranean climate is one of the most pleasant for
humans, especially near coasts, where cooling ocean breezes
moderate the summer extremes. Before modern transportation and irrigation schemes, local sustainability had its limits,
but today, largely because of the importation of water, huge numbers of people have migrated to the cities of these regions. This
vast influx has had disastrous effects on the local plant life, mitigated only by the fact that steep topography, rocky soils, and a
lack of access to imported water have put limits on agriculture
and urban expansion. In less affluent areas, domestic grazing and
browsing have reduced the native component of vegetation and
increased the proportion of weedy species, many of them significantly less palatable because they are spiny or poisonous.
Wildfire is a continuing problem in Mediterranean areas, the
problem being proportional to the size of the human population,
which increases the points of contact between people and fires.
Also important is the state of the vegetation—that is, whether
land use allows the shrub vegetation to approximate its naturally
dense and continuous growth. Any wildfire is dangerous, but fires
in dense shrublands are especially so. The heat generated by a
wildfire can be fatal to humans, even if they are only near an
area that is burning. In the windy conditions common in wildfires, houses can be ignited hundreds of meters from the flames
when burning embers are lofted into suburban developments.
Furthermore, wildfires that spread from a human ignition source
(arson, campfires, fallen power lines, sparks thrown by mowing
machines, and many more sources) can be more frequent than
those arising from natural ignitions—mostly lightning and blowups from lightning-caused smoldering fires.
As long as there is a natural cover of shrubs or grasses,
fire cannot be eliminated from the landscape, but it is commonly asserted that the problem is made much worse by past
management policies. The argument is that in attempting to banish fire, we have favored biomass accumulation and therefore
larger and more damaging fires. There is some truth to this, but
the simple solution of suspending fire suppression is not feasible.
Restoring fire to its “natural role” requires first reducing current
fire hazards and then implementing a scheme of regular burning
or something that simulates burning. Both of these solutions are
costly and require careful planning. The giant wildfires of recent
years suggest that we have not even begun to achieve this hypothetical benign fire landscape.
The Northernmost Forests—
Taiga and Boreal Forest
Moving poleward from the mixed coniferous forest, the seasonal contrast becomes more pronounced, with very short and
extremely cold days in winter and a short growing season of
long days. In these conditions, trees still dominate, because long
days during the brief summer permit them to accumulate enough
energy to overtop competing life forms. These northern coniferous forests are often referred to by the Russian name taiga; in
North America they are also called the boreal, or northern, forest. Taiga extends over much of Russia, Scandinavia, and northern North America (Figure 32–2). Throughout its range it is
characterized by a persistent cover of snow in the winter. In the
southern reaches of the taiga, the trees are taller and more luxuriant, often reaching 75 meters or more in height. In its main
northern area, however, the trees are shorter, and thousands of
square kilometers are covered by this uniform forest, with relatively few species of plants and animals (Figure 32–26).
Taiga occurs in the interior of large continental masses. In
such regions, extreme temperatures range from –50° to 35°C.
32–24
C ha p t e r 3 2 Global Ecology
32–26 Northern taiga The northern
taiga, which covers hundreds of
thousands of square kilometers in the
cooler part of the north temperate zone,
is dominated by white spruce (Picea
glauca) and larch (Larix), a deciduous
conifer. This photograph was taken in
northern Manitoba, Canada. In the more
northern part of their range, the trees are
smaller than seen here.
Taiga is flanked on the south by montane forests (as in western North America), deciduous forests, savannas, or grassland,
depending on the amount of precipitation in the region. Because
continental masses do not occur at the appropriate latitudes in the
Southern Hemisphere, taiga is absent there. Due to the influence
of the prevailing westerlies blowing over relatively warm ocean
currents between 40° and 50° north latitude, the western portions of North America and Eurasia are characterized by milder
climates than their eastern portions. Consequently, taiga is found
somewhat farther north toward the Pacific coast than it is along
the Atlantic coast in North America, and the same is true of the
individual distributions of many kinds of plants and animals that
inhabit the biome.
The northern limit of taiga is determined by the severity of
the Arctic climate, and it roughly corresponds to where the maximum monthly temperature is approximately 10°C. At its northern limits, taiga grades unevenly into tundra. In the extensive
northern reaches of the taiga, most of the precipitation falls in
the summer; the cold winter air in these regions has a low moisture content. The annual total precipitation usually amounts to
less than 30 centimeters. Despite the low rainfall, it is a region of
moisture surplus because of low evapotranspiration. Lakes, bogs,
and marshes are common (Figure 32–26).
A major shift occurs with the appearance of permafrost. In
less severely cold climates, the zone of freezing moves downward
from the surface, reaching a maximum depth, and then thaws
from both the bottom and the top as the weather warms. But in
more than three-quarters of the northern reaches of the taiga, this
summer thawing does not extend through the frozen zone before
the onset of winter, thus creating a permanently frozen zone, or
permafrost.
Permafrost has many ecological consequences. Since water
cannot move through the permafrost, and water will be released
in thawing, saturated conditions and the presence of ponds are
favored. Permafrost provides a stable base for soils that are often
high in organic matter. When thawing eliminates permafrost
or descends to deeper levels, the surface soils can be destabilized, causing trees to tip and even fall over in “drunken forests”
(page 32–1). Houses and other buildings can also be damaged or
destroyed when the permafrost thaws.
In general, taiga soils are highly acidic and very low in
nutrients. Species of a few genera of trees are common in the
northern taiga, including spruce (Picea), larch (Larix), fir
(Abies), and poplar (Populus). Among the more common shrubs
are dewberries (Rubus), Labrador tea (Rhododendron), willows
(Salix), birches (Betula), and alders (Alnus). Pines (Pinus) can
be common, usually in drier areas. The members of all these
genera of trees and shrubs are ectomycorrhizal (pages 312 to
314, Figure 29–1), and they grow in dense stands consisting of
only one or a very few species. Perennial herbs are common,
but annual plants, except in areas disturbed by humans, are
essentially absent. Nonvascular plants—mosses and lichens—
play an important role in the taiga and other far poleward
communities. It is common in these forests to have dense and
continuous layers of mosses and lichens, often many centimeters thick (Figure 32–27). These plants have both positive and
negative influences on their neighboring vascular plants. In
extreme cases, mosses, especially Sphagnum species, can convert forest into bog by trapping rainfall and creating thick, moist
mats that discourage establishment of trees.
Because of the seasonally abundant light and favorable temperatures, cool-season cultivated plants, such as cabbages (Brassica oleracea var. capitata), may grow rapidly in cleared areas in
the taiga, attaining large sizes in a remarkably short time. Yet the
infertile, highly leached soils of the taiga do not allow most forms
of agriculture.
Arctic Tundra 3 2 – 25
local melting of the permafrost. But as proof that generalities
have notable exceptions, there is also a genus of deciduous conifer, the larches (Larix spp.), that is widely distributed throughout
the boreal regions and often dominant. Its success appears to be
due to a combination of the superior cold-tolerance of conifers
with an improved ability, attributable to its deciduousness, to
exploit more favorable nutrient conditions.
Coniferous forests extend far to the south of taiga, along
the Pacific coast of North America. These magnificent evergreen
coniferous forests (Figure 32–2) occur where there is a pronounced summer drought, but high and persistent rainfall during
the cooler seasons. Because photosynthesis is limited by lack of
moisture during the warm season, deciduous trees are at a disadvantage and are usually found only along stream banks. The
evergreen conifers, however, can synthesize carbohydrates all
year round and, because of their massive size, can store water and
nutrients for use during the dry season.
Arctic Tundra
32–27 Understory vegetation Taiga, such as this forest in Lac La
Ronge Provincial Park in Saskatchewan, frequently has understory
vegetation dominated by continuous mats of mosses and lichens.
These nonvascular plants affect many processes, especially plant
regeneration, nutrient cycling, and depth of thaw of the permafrost.
The abundance of evergreen conifers raises a question: why
is it advantageous to be evergreen in a climate that has such a
long cold season? It is presumed (and partly proven) that the
shortness of the growing season and the generally low fertility of
boreal soils make the deciduous habit less efficient. During the
short season with a less predictable start to a frost-free summer,
unfolding a new leaf from a dormant bud is less efficient than
retaining leaves that can immediately resume photosynthesis.
Further, the cool temperatures tend to slow down the decomposition and recycling of nutrients. As nutrients become more limiting, a leaf that functions over more than one season is capable of
more photosynthesis per unit of nutrient absorbed. Broad-leaved
deciduous trees do occur in the most northern forests, but they
often are restricted to situations such as the margins of streams,
where frost does not penetrate so deeply into the ground, or in
the early nutrient-rich stages of succession after fire. Fires are
common in the taiga, and they result in generally warmer, more
productive sites for at least 10 to 20 years afterward, due to the
The Arctic tundra is a treeless biome that extends to the farthest
northern limits of plant growth (Figure 32–28). It occupies an
enormous area: fully one-fifth of the Earth’s land surface (Figure
32–2). Many species of plants that grow in the Arctic tundra have
wide circumpolar ranges. As the name suggests, Arctic tundra lies
mostly above the Arctic Circle, extending farther south along the
eastern sides of the continents than along their western sides. The
Arctic tundra essentially constitutes one huge band across Eurasia
and North America, with alpine tundra, more closely related to
the adjacent mountain forests, extending southward in the mountains (Figure 32–22; see also Figure 32–5). Tundra vegetation
exists in the Southern Hemisphere, but it is not well developed,
given the limited land area available at the appropriate latitudes.
Tundra-like vegetation also occurs at the highest elevations of
mountains.
In the tundra biome, the seasonal variation in climate is
extreme. Although daylength increases in summer—north of the
Arctic Circle, the sun does not set for days—this is balanced
by a dark winter. This portion of the Earth operates at a strong
solar energy deficit, radiating more energy back to space than it
receives in direct sunlight. Frosts can occur throughout the year,
and winters are bitterly cold, with high winds that will desiccate any plant that rises above the snow level. At low temperatures, blowing snow acts as an abrasive that ruthlessly attacks any
exposed tissues. During the summer, the limited supply of solar
energy is insufficient to thaw more than a shallow layer of soil
above the permafrost. The annual freezing and thawing of the
surface has many effects unknown in milder climates, such as
landscapes broken into polygonal blocks ranging from 3 to 30
meters across and small ice-cored hills (pingos) that rise up to
50 meters high. The soils are acidic to neutral and low in nutrients. Even though precipitation is usually less than 25 centimeters per year, much of it is held near the surface by underlying
permafrost, and on more level areas the ground is usually wet,
with many small pools. Drier, usually rocky upland areas are also
present. Because there are only a few species of legumes and
other plants with symbiotic bacteria that fix atmospheric nitrogen,
available nitrogen is often in short supply in the soil.
32–26
C ha p t e r 3 2 Global Ecology
(a)
(b)
32–28 Arctic tundra (a) Arctic wet coastal tundra near Prudhoe Bay, Alaska, in late summer. The reddishbrown plants are the grass Arctophila fulva, the green ones are the sedge Carex aquatilis. Note that the
water table is above the surface because of the presence of permafrost; such conditions are characteristic
of the Arctic tundra. (b) Arctic tundra at Barrow, Alaska. A clone of the sedge Eriophorum angustifolium is
growing out into a solid patch of another species of the same genus, Eriophorum scheuchzeri. Vegetative
reproduction, as in E. angustifolium illustrated here, is characteristic of many tundra plants.
The vegetation over such a vast area has many variants, with
the moisture conditions of the substrate an important factor. There
are often distinct plant assemblages alternating on a small scale
from pond to dry rocky hilltops. Another variable is the length of
time that snow remains. Areas that receive less solar radiation and
accumulate deeper snow will thaw later, so the growing season
can differ significantly in the space of only a few meters. More
exposed rocky areas, which tend to be blown free of snow in the
winter, have a low cover of sedges, grasses, and dwarf, mostly
evergreen shrubs. Both lichens and mosses are present in abundance, with more moss in wetter places and more lichens in drier
spots, especially on the surfaces of exposed rocks and boulders.
Several genera of low shrubs, including birch (Betula), willow
(Salix), blueberry (Vaccinium), and Labrador tea (Rhododendron),
are common. Shrubs such as willows often occupy lower positions, with their canopies at roughly the same height as the surrounding, more herbaceous vegetation. This is due to the action
of the freeze-drying winds of winter. Annual species are rare,
with the widely distributed Koenigia islandica an exception (Figure 32–29a). The high proportion of evergreen species in the tundra can be explained by the same factors that prevail in the taiga.
Nutrients are limited, and the brief growing season places a premium on photosynthesizing as soon as possible and intermittently
between cold periods. For similar reasons, vegetative propagation
is common and establishment by seed relatively rare. Much of the
biomass of tundra plants—from about 50 percent to as much as
98 percent—is underground, consisting of roots and underground
stems. A number of tundra plants have relatively large, showy
flowers, the production of which costs the plants considerable
quantities of energy (Figure 32–29b). Such flowers hold energyrich rewards for their pollinators—necessary rewards, given the
low temperatures that prevail at high latitudes.
North of the Arctic tundra is ice desert, where physical conditions are so extreme that vegetation is essentially absent. Ice
desert is characteristic of the interior of Greenland; of Svalbard,
a small group of islands off Norway, one of which is the site of
the Global Seed Vault (page 677); and of Novaya Zemlya, two
islands off the north coast of Siberia. Much of Antarctica, not
mapped in Figure 32–2, is also covered by ice or by Arctic deserts with patches of mosses and lichens. If projections of climatologists prove true, we can expect a considerable diminution of
the ice cover of polar, and especially Arctic, regions.
A Final Word
What will be left of the natural biosphere in 2100? According
to the United Nations, the population of the world will peak at
9 to 10 billion people in 2100. We are going to find out which
bumper-sticker sentiment is closer to the truth: “More people
means more hands to work and brains to think” or “More people means more mouths to feed, more pollution and land degradation.” For human welfare, it will matter a great deal which
of these best describes the future. To a conservationist, it is not
clear that there is a lot of difference between them. History tells
us that when pressure from the human population increases, there
is further exploitation of the natural world. If we are optimistic
conservationists, we can hope for the first outcome. Perhaps one
of the “extra” 3 billion people will be an agronomical genius
who devises sustainable schemes to double the world’s agricultural output. Then it would be possible to hold the line on further destruction of natural habitats. But even if this worldwide
transformation of agriculture could be achieved, it would still be
true that 42 percent more people would be encroaching on wild
habitats, building more houses, roads, and trails, and demanding
Summary 3 2 – 27
(a)
(b)
32–29 Tundra flowers (a) Koenigia islandica, also known as Iceland, or island, purslane, grows on damp rocky
areas, especially near persistent patches of snow, in the tundra and alpine meadows of the Northern and Southern
Hemispheres. (b) Mountain avens (Dryas octopetala), a large-flowered Arctic plant of wide distribution, is seen here
growing in Siberia. Its pollen has been used as a marker, or indicator, of colder climates, and its name has been
adopted to describe periods of cooling during glacial times, as in “the younger and older Dryas periods.”
the control of dangerous or annoying wild animals. If the pessimists are correct, and more people means more threats to the biosphere, the future for conservation is even bleaker. If we increase
agricultural productivity 10 percent instead of 100 percent, and
simultaneously need to produce biomass energy, much more
land will be needed. And this additional land will come primarily from places where human populations are low—that is, from
whatever remains of our natural landscapes. If the more dire predictions of climate scientists prove true, there will be further disruptions and instabilities.
What is the appropriate response of a conservationist to this
possibility of a bleak future? Given the intense pressure from the
disdainers of wild nature, it is clear that to save as much of the
natural part of the biosphere as possible, we will need continuing
counter-efforts across a broad front. There are utilitarian as well as
aesthetic and moral reasons for not destroying nature. We need all
of these arguments—and articulate persons to make them. But as
believers in science (which we expect you are, too), we recognize
the critical importance of a scientific understanding of how the
world ecosystems work. We hope this brief overview of Earth’s
natural ecosystems will give you a foundation on which to build
and will motivate you to do your part in saving the natural world.
SUmmary
Vegetation Differs from Place to Place and from Time to
Time, but Large-Scale Patterns Are Discernible
On a small scale, vegetation is highly variable because of patterns of past disturbance and differences in the substrate. But
when viewed on a larger scale, vegetation separated by many
thousands of kilometers can appear very similar. This similarity can arise because the species are closely related or because
similar climates and substrates in different areas favor convergent evolution, or both.
Biomes Are Terrestrial Ecosystems Characterized by
Distinctive Vegetation
Biomes are more or less homogeneous regional or global ecosystems characterized by plant species that share physical and
physiological traits. The similarity in species traits arises primarily because of the similarity in climate. Because they cover broad
areas, biomes also contain local variation, areas that differ from
the defining communities. So, for example, the temperate deciduous forest biome includes extensive herbaceous marshes.
Tropical Rainforests Have a Great Diversity of
Species
Tropical rainforests, with their consistently warm temperatures
and ample moisture, are the richest biomes in terms of number of
species. The trees are predominantly evergreen and broad-leaved.
A poorly developed layer of herbaceous plants grows on the forest floor, but there are many vines and epiphytes at higher levels.
Tropical soils are often acidic and very poor in nutrients; such
soils lose their fertility rapidly when the forest is cleared.
Savannas and Deciduous Tropical Forests Occur Where
Rainfall Is Seasonal
Mostly tropical and subtropical communities that are characterized by a seasonal drought are termed savannas, subtropical mixed forests, monsoon forests, tropical mixed forests, and
southern woodland and scrub. The trees and shrubs of these
communities are wholly or partly deciduous, shedding their
leaves during times of drought. Herbaceous perennials are common. Savannas also occur between the prairies and temperate deciduous forests and between the prairies and the taiga in
North America. Subtropical mixed forests cover most of Florida
and the coastal plain of the southeastern United States. In these
forests, pines and other evergreen trees grow intermixed with
deciduous trees.
32–28
C ha p t e r 3 2 Global Ecology
S u mmar y T abl e Some Characteristics of the Earth’s Principal Biomes
B IO M E
T E M PE R AT U R E A N D P R E C IPI TAT IO N Rainforests
High temperature and high rainfall year round
CHARACTERISTIC PLANTS
M I S C E L L A N EOU S F E AT U R E S
Broad-leaved evergreen trees, The biome with the greatest diversity of species;
epiphytes, and lianas
infertile soils
Savannas and High temperature and seasonal drought
Grasslands with scattered broad-leavedPeriodic burning is an important aspect
deciduous tropical deciduous or evergreen trees or shrubs
forests
DesertsPrecipitation generally very low except for occasional peaks; maximum temperature varies with the type of desert
In warmer, slightly wetter deserts,
succulents such as cacti; in all deserts,
deep-rooted drought-resisting shrubs and annual plants that flourish after rare rains
Grasslands
Moderately low precipitation, cold winters, and Perennial bunchgrasses and sod-
warm summers
forming grasses
Temperate deciduous forests
Moderate precipitation evenly distributed, cool
winters, and warm summers
Adaptations include small leaves, thick cuticles,
and photosynthetic rates with high midsummer
temperatures; spacing of dominant perennial
plants reflects competition for underground water
more than for light
Heavily exploited for agriculture or for grazing of
domestic animals
Deciduous trees and many perennialThe dominant herbaceous plants vary with the
herbs
seasons
Temperate mixed Moderately low precipitation and moderately
Mixtures of deciduous trees and
and coniferous cold winters
conifers
forests
Occur as a transition zone north of the deciduous
forest; also found in areas with nutrient-poor
soils or with less seasonal environments
Mediterranean
Cool, moist winters and hot, dry summers
scrub
Evergreen or summer-deciduous
drought-resistant trees and shrubs in
dense thickets
Called chaparral in California and maquis around
the Mediterranean Sea
Taiga
Moderately low precipitation and cold winters
Forests mostly of conifers, mostly evergreen but with a significant component of deciduous Larix species
in some places
Soils are highly acidic and very low in nutrients;
permafrost may be present
Arctic tundra
Low shrubs, grasses, sedges, andPermafrost present throughout; much of the
lichens
biomass is underground
Very low precipitation in both summer and winter, very cold winters
Desert Plants Are Adapted to Low Precipitation and
Extremes of Temperature
Away from the equator, tropical and subtropical communities
grade into deserts and semideserts, which are characterized by
low precipitation and often by high daytime temperatures during at least part of the year. Succulent plants are common in the
warmer and less extreme deserts, and drought-tolerant smallleaved shrubs are found in most deserts.
Grasslands Occur Where Precipitation and Fire Interact
to Support Grasses but Not Trees
Grasslands, which intergrade with savannas, deserts, and temperate forests, are characterized by a general lack of trees, except
along streams. The height and density of the grass-dominated
cover varies from scattered tussocks in desert grasslands to shortgrass turfs, to mid- and tallgrass prairies, where rainfall is greatest. Grasslands favor the accumulation of organic matter, often
to considerable depths. The most productive soils for temperate
agriculture are grassland soils.
Temperate Deciduous Forests Are Made up of LeafShedding Trees and Many Types of Perennial Herbs
In the temperate deciduous forests, most of the trees lose their
leaves during the cold (usually snowy) winters, when moisture
may be unavailable for growth. Many genera are common to the
temperate deciduous forests of eastern North America, northern Europe, and eastern Asia. Temperate deciduous forests are
bordered by temperate mixed and coniferous forests to the north,
in which conifers play an important role.
Mediterranean Climate Scrub Is Characterized by
Evergreen, Drought-Resistant Shrubs or Trees That Form
Thickets
Distinctive shrub communities, called chaparral in North America and maquis in the Mediterranean region, have evolved in the
five widely separated areas of the world with a Mediterranean
climate—a dry summer and a cool, rainy winter growing season.
Such communities occur in western North and South America,
around the Mediterranean, in the Cape Region of South Africa,
and in southwestern Australia. In all regions they are subject to
more or less frequent intense wildfires.
The Taiga Is Characterized by Forests of Coniferous Trees
The taiga is a vast northern coniferous forest that extends in
unbroken bands across Eurasia and North America. In its southern reaches, the taiga is dominated by tall trees with a lush
growth of mosses and lichens; northward, it consists of vast
monotonous stretches of forest with very few tree species.
The Arctic Tundra Has Low-Lying Shrubs and Grasses
but No Trees
Tundra occurs poleward of the evergreen taiga-type forests,
where the intense winter cold and short growing season preclude
tree growth. It extends around the Northern Hemisphere, mostly
PASS 1
Photo Credits 3 2 – 29
above the Arctic Circle, in a band that is broken only by bodies of
water. Both the northern reaches of the taiga and all of the tundra
are underlain by permafrost. Because of this permafrost, and especially because of the low rates of evapotranspiration, tundra and
taiga soils are relatively moist and highly leached of nutrients.
The Earth’s Biodiversity Is under Siege, and Efforts Must
Be Made to Protect It
The growing human population continues to diminish our global
stock of natural areas. If we are to save enough of it to prevent
massive extinctions, we must redouble our efforts at conservation
and restoration.
QUESTIONS
1.Describe the influence of latitude and altitude on the distribution
of organisms on Earth.
2.Describe the effect of mountains on local precipitation.
3.Compare tropical and temperate forests in terms of the numbers
of species found in each, and in the size and appearance of the
trees.
4.Explain why annual plants are better represented, both in number
and in kind, in the deserts and semiarid regions of the world than
anywhere else.
5.Compare the relative amounts of nutrients found in forest and
grassland soils.
6.How are the evergreen conifers of the Pacific Northwest of the
United States and Canada adapted to the winter-wet, summer-dry
environment of that region?
7. What are the principal differences between taiga and tundra?
What role does permafrost play in these biomes?
8. The concept of “ecological services”—benefits that humans
derive from the existence of natural systems—has become a
guiding idea in recent decades. What are these benefits, and how
are they likely to change as the effects of human interference
continue to increase?
9.Using Google Earth, find the latitude of your current location.
Follow this latitude around the globe, zooming in at enough
points to get an impression of how the vegetation changes. What
causes the changes you observe? What proportion of the surface
shows clear evidence of human activity? How much looks more
or less natural?
PHOTO CREDITS
Chapter Opener Ashley Cooper, Visuals Unlimited/Science Photo
Library; 32.1a Wikipedia Commons; 32.1b Martin Zeick, Age
Fotostock; 32.2 After A. W. Küchler; page 32-8 Institut für
Auslandbeziehungen, Stuttgart, Germany; 32.7a Michael Fogden/
DRK Photo; 32.7b Martin Wendler/NHPA; 32.7c E. S. Ross; 32.8
Frans Lanting/Minden Pictures; 32.9 Peter Ward/Bruce Coleman,
Inc.; 32.10a, b J. Reveal; 32.11a Martin Wendler/NHPA; 32.11b
Creative Commons, photo by Stan Shebs; 32.11c Ric Ergenbright;
32.11d Greg Vaughn/Tom Stack and Associates; 32.12 Corbis RF/
Alamy; 32.13 F. C. Vasek; 32.14 Ray Evert; page 32-17 (both)
Ronald F. Thomas/Bruce Coleman, Inc.; 32.15a Jeff Foott; 32.15b
Pat Caulfield; 32.16 Soil Conservation Service; 32.17a Rod Planck/
Tom Stack and Associates; 32.17b P. White; 32.18 (all) J. H.
Gerard; 32.19 R. Burda/Taurus Photos, Inc.; 32.20 J. Dermid; 32.21
Tom Salyer/Alamy; 32.22 E. Beals; 32.23 E. S. Ross; 32.24 David
Woodfall/WWI/Peter Arnold, Inc.; 32.25a Jack Wilburn/Earth
Scenes; 32.25b Ardea Photographics; 32.26 J. Bartlett and
D. Bartlett/Bruce Coleman, Inc.; 32.27 Ron Erwin Photography;
32.28a, b W. D. Bellings; 32.29a Bob Gibbons/Science Photo
Library; 32.29b Serg Zastavkin, Shutterstock Images